The present invention generally relates to focusing control systems of optical recording/reproducing apparatus recording and/or reproducing an information signal on and from a rotary information recording medium by means of an optical beam, and in particular to an optical system used in such a focusing control system for focusing the optical beam on the rotary information recording medium.
An optical recording/reproducing apparatus such as a magneto-optical recording/reproducing apparatus records and/or reproduces an information signal on and from a rotary information recording medium referred to hereinafter as a disk by means of an optical beam. The optical beam is irradiated on the disk through an objective lens held in the apparatus and the disk is rotated together with a turntable which supports the disk. In such an apparatus, it is impossible to eliminate minute displacement of the disk in the upward and downward direction when the disk is rotated together with the turntable, and a proper focusing of the optical beam on a recording plane of the disk on which the information signal is recorded cannot be maintained as long as the objective lens is fixed in the apparatus. Therefore, the apparatus uses a focusing control system which maintains the focusing of the optical beam by moving the objective lens to and from the disk responsive to the distance between the objective lens and the recording plane of the disk such that the distance between the objective lens and the recording plane of the disk is maintained equal to the focal length of the objective lens. The focusing control system includes an optical system which in turn comprises a plano-convex lens held by a first holder, a cylindrical lens held by a second holder and a photo diode array having a quadrant detecting plane, and a reflected optical beam reflected by the disk is focused on the quadrant detecting plane after passing through the plano-convex lens and the cylindrical lens. The plano-convex lens and the cylindrical lens are positioned relative to the quadrant detecting plane such that the reflected optical beam reflected by the disk and received by the objective lens forms a circular beam spot on the detecting plane when the optical beam is properly focused on the disk by the objective lens. In case that the distance between the disk and the objective lens is not equal to the focal length of the objective lens and the optical beam is not properly focused on the disk, the spot of the optical beam formed on the quadrant detecting plane of the photo diode array assumes an ecliptical shape elongating in one or the other of the diagonal directions defined in the quadrant detecting plane. Responsive to the appearance of this elongated beam spot on the detecting plane, the photo diode array cooperating with the quadrant detecting plane produces an electrical output indicating that the objective lens is located too close to or too far from the recording plane of the disk. This control signal is supplied to a focusing servo control system which moves the objective lens to and from the disk responsive to the electrical output produced by the photo diode array. Thus, a proper focusing of the optical beam is recovered even when the disk is displaced upwards or downwards and the focusing of the optical beam is lost.
In such a conventional focusing control system, one has to:
1) dispose the plano-convex lens, cylindrical lens and the quadrant detecting plane of the photo diode array on a single optical path such that optical axes of these elements coincide each other;
2) adjust the position of the plano-convex lens, cylindrical lens and the quadrant detecting plane such that a circular beam spot appears on the quadrant detecting plane when the optical beam irradiated on the disk via the objective lens is properly focused on the recording place of the disk; and
3) rotate the cylindrical lens about its optical axis such that the direction of elongation of the ecliptical optical beam spot on the quadrant detecting plane which appears when the proper focusing of the optical beam on the disk is lost coincides with the one or the other of the diagonal lines of the quadrant detecting plane, when assembling the optical system of the focusing control system. Here, the optical axis of the quadrant detecting plane is defined as a hypothetical axis passing through a center of four detecting windows of photo diodes arranged in a row and column formation perpendicularly to the plane of the detecting window.
In such an adjustment, it is relatively easy to establish a proper relation between a pair of optical elements such as the plano-convex lens and the cylindrical lens, the cylindrical lens and the quadrant detecting plane, or the plano-convex lens and the quadrant detecting plane. However, it is extremely difficult to adjust the relation between the plano-convex lens, the cylindrical lens, and the quadrant detecting plane in a single step. Thus, when aligning the optical axes of the plano-convex lens, cylindrical lens and the quadrant detecting plane of the optical system, the optical axes of the quadrant detecting plane and the cylindrical lens are aligned first, and then the optical axes of the cylindrical lens and the plano-convex lens are aligned. In this state, the optical axes of the quadrant detecting plane and the plano-convex lens do not generally coincide one another. Thus, it is necessary to coincide the optical axes of the plano-convex lens and the quadrant detecting plane in the following step. As the optical axes of the quadrant detecting plane and the cylindrical lens are aligned at the beginning of the adjustment, this adjustment of the quadrant detecting plane and the plano-convex lens generally may lead to the misalignment of the optical axes of the quadrant detecting plane and the cylindrical lens. Therefore, the alignment of the optical axes of the quadrant detecting plane and the cylindrical lens has to be established again, and the above procedures for achieving the alignment have to be repeated until a satisfactory alignment of the overall optical system is achieved. Similarly, the position of these optical elements are adjusted repeatedly. Further, it is necessary to adjust the orientation of the cylindrical lens such that the elongated beam spot appearing on the quadrant focusing plane of the photo detector array coincides with the direction of the diagonal line of the detecting plane. Thus, the cylindrical lens is rotated relative to the holder holding the lens and is fixed with the orientation thus established.
Thus, conventional optical system of the focusing control system used in the optical recording/reproducing apparatus has a problem that a repetitive and complex adjustment is necessary, and the assembling work is tedious and time consuming, and expensive.